1. Rinse the beef liver under cold water and pat dry. Trim any connective tissue and slice into 1/2-inch thick pieces. Lightly dredge each piece in flour, shaking off the excess.

2. In a large skillet, melt 2 tablespoons of butter with 1 tablespoon olive oil over medium heat. Add the sliced onions and cook slowly, stirring occasionally, for 20-25 minutes until deeply golden brown. Remove and set aside.

3. In the same skillet, add 1 tablespoon of butter and 1 tablespoon olive oil. Add the floured liver slices in a single layer and cook for 2-3 minutes per side, until browned but slightly pink in the center. Remove and set aside.

4. Add minced garlic to the skillet and sauté for 30 seconds. Pour in beef broth and Worcestershire sauce. Scrape the pan to deglaze and combine flavors.

5. Return the onions and liver to the skillet. Simmer for 3-5 minutes to allow flavors to meld and the sauce to slightly thicken.

6. Garnish with chopped parsley and rosemary if desired. Serve hot with mashed potatoes, rice, or crusty bread.